Secondary batteries are widely used in devices ranging from electronic devices such as cellular phones, notebook computers, digital cameras, and camcorders to electric vehicles or hybrid vehicles. Unlike primary batteries, secondary batteries can be repeatedly recharged. Particularly, with rapid technological advances, smaller and lighter large-capacity and high-power secondary batteries have recently been developed.
Such secondary batteries can be manufactured in a variety of forms, having, for example, a pouch, cylindrical, or polygonal shape.
For a pouch-type secondary battery, a pouch casing has a multilayered structure that includes a thin metal film and insulating films attached to both sides of the thin metal film, and is flexible, unlike a cylindrical or polygonal secondary battery made of a comparatively thick metal plate. Moreover, the pouch-type secondary battery is light and small compared to those made of metal, and can be easily adapted to the desired battery shape, or to suit the method of installing the battery in an electronic device or the like. In light thereof, the pouch-type secondary battery has many advantages as a secondary battery.
To produce such a pouch-type secondary battery, an electrode assembly and electrolyte are put in the pouch casing, and then vacuum pressure is applied to the pouch casing before the pouch casing is sealed. Electrode tabs are connected to the electrode assembly and extend outward from the pouch casing so as to electrically connect the electrode assembly to an external circuit.
The electrode assembly housed in the pouch casing is made of a multilayered film that includes a positive plate, a separator, and a negative plate that are successively placed on top of one another. The electrode assembly may be provided to have a form of a jelly roll, formed by winding the multilayered film. For the jelly roll, a separator is provided on the surface of an electrode that is exposed to the outside, thus preventing a short circuit between the positive plate and the negative plate when the jelly roll is wound.
The conventional pouch-type secondary battery is, however, disadvantageous in that, when a vacuum is applied to the pouch casing with the electrode assembly disposed in the pouch casing, the corners of the forming part in which the jelly roll is disposed are easily crushed and deformed.
One reason for this is that the separator, which is thin and is made of material having comparatively low hardness, is disposed around the most peripheral portion of the jelly roll. Another reason is that the pouch casing itself is made of flexible material having comparatively low stiffness, which is thus not able to reliably protect the electrode assembly.